1. Technical Field
The present invention relates to a technology for controlling behaviors of a variety of electro-optical elements such as light-emitting elements made of organic electroluminescent materials.
2. Related Art
In such electro-optical elements, a level of gray scale (typically referred to as brightness) is changed according to on a supplied current. There has been suggested a structure in which the current (hereinafter referred to as “driving current”) is controlled using a transistor (hereinafter referred to as “driving transistor”). However, there is a problem with such a structure in that unevenness in gray-scale level of light-emitting elements attributable to different properties (in particular, threshold voltages) of driving transistors occurs. In order to solve the gray-scale unevenness problem, U.S. Pat. No. 6,229,506 (FIG. 2), JP-A-2004-133240 (FIGS. 2 and 3) and JP-A-2004-246204 (FIGS. 5 and 6) disclose structures which are capable of compensating variations in threshold voltages of driving transistors.
FIG. 16 shows a pixel circuit P0 disclosed in U.S. Pat. No. 6,229,506. As shown in this figure, a transistor Tr1 is interposed between a gate and a drain of a drive transistor Tdr. The gate of the drive transistor Tdr is connected to a first electrode L2 of a capacitive element C0. A storage capacitor C1 is a capacitor interposed between the gate and a source of the drive transistor Tdr. A transistor Tr2 is a switching element interposed between the data line 14 and the second electrode L1 of the capacitive element C0, thereby controlling electrical connection and disconnection between the data line 14 and the second electrode L1 of the capacitive element C0, where the data line is supplied with a potential Vd (hereinafter referred to as “data potential”) corresponding to a brightness set in an organic light-emitting diode element (OLED element) 110.
The aforementioned pixel circuit P0 operates in the following manner. First the transistor Tr1 is transited to an on-state by a signal S2, and thus the drive transistor Tdr is connected as a diode. At this time, a potential of the gate of the drive transistor Tdr converges to a value of “VEL−Vth” (Vth is a threshold voltage of the drive transistor Tdr). Second, the transistor Tr 2 is turned on by a signal S1 in the off-state of the transistor Tr1, and thus the electrode L1 of the capacitive element C0 and the data line 14 are electrically connected to each other. Through this operation, the potential of the gate of the drive transistor Tdr changes by a level (a level corresponding to the data potential VD) which is a value calculated by dividing a potential variation of the electrode L1 by a capacitance ratio of a capacitance of the capacitive element C0 to a capacitance of the storage capacitor C1. Third, a transistor Tel is turned on by a signal S3 when the transistor Tr2 is in the off-state. As a result, a driving current Iel which does not depend on the threshold voltage of the drive transistor is supplied to the OLED element 110 via the drive transistors Tdr and Tel. The basic principle for compensating the threshold voltage Vth of the driving transistor Tdr disclosed in the aforementioned structure is the same in the examples of the related art disclosed in JP-A-2004-133240, and JP-A-2004-246204.
In all the structures disclosed in U.S. Pat. No. 6,229,506, JP-A-2004-133240, and JP-A-2004-246204, the electrode L1 of the capacitive element C0 is in the electrically floating state due to the transition of the transistor Tr2 to the off-state during a period (hereinafter, referred to as “light-emitting period”) in which the OLED element 110 actually emits light. Accordingly, a voltage of the capacitive element C0 is liable to fluctuate. For example, there is a probability that a potential of the electrode L2 changes due to noise attributable to switching operations of the transistor Tr2. As described above, when the voltage of the capacitive element C0 changes during the light-emitting period, the potential of the gate of the drive transistor Tdr or the driving current Iel corresponding to the potential of the gate of the drive transistor Tdr likewise changes. As a result, unevenness of brightness (unevenness of a display, such as crosstalk) of the OLED elements 110 occurs.
On the other hand, there is a technique of decreasing influence of the potential variation of the electrode L1 on the potential of the gate of the drive transistor Tdr. That is, when the capacitance of the capacitive element C0 or the storage capacitance C1 is increased, the potential of the gate of the drive transistor Tdr may be less affected by the potential fluctuation of the electrode L1. However, this method is accompanied with other problems having to be increased in the size of the pixel circuit P0 because it is required that the capacitance be increased. Accordingly, this method cannot be a practical solution under circumstances in which fine pixels are highly demanded.